A THEORETICAL FRAMEWORK FOR CONCURRENT INTEGRATION OF CAD WITH DOWNSTREAM APPLICATIONS.pdf

advances in production engineering for instance shoulder and boss are examples of form features which are significant during design and manufacturing 29 since form features can represent at least some aspects of design intent and part functionality many research efforts have been conducted in techniques for deriving form feature information from a geometric database where form features are recognized extracted and classified using algorithms design features design features such as protrusion slot hole rib and slot are defined as a combination of abstract form features and design intents the design intents include material features 208 ullah a theoretical framework for concurrent integration of cad with downstream applications precision features and functional features material features specify material composition and condition information such as properties specifications or treatment applied to materials and surfaces precision features specify geometric dimensions and tolerances and surface finish functional features specify performance parameters operating variables or design constraints 11 machining features a machining feature is defined as a shape that represents volumes to be removed by machining to obtain the final part geometry from the initial stock 1 figure 1 shows the machining features classification defined in step ap224 figure 1 machining features classification by step assembly features assembly feature is defined as a connection between two form features on mating parts associated with assembly intents where assembly intents include assembly relation assembly operation and assembly degrees of freedom 14 as shown in figure 2 table 1 shows examples of assembly features 209 ullah a theoretical framework for concurrent integration of cad with downstream applications figure 2 assembly feature definition table i examples of assembly features assembly features can be used as a main factor in the integration of cad with assembly planning because various design and assembly data can be associated with assembly feature the integration of design and assembly planning is very important for achieving assembly oriented product development to reduce manufacturing cost and enhance the 210 ullah a theoretical framework for concurrent integration of cad with downstream applications production efficiency and product quality however most of existing feature based assembly research is displaying information to guide assembly modeling planning and integration but there is lack of detailed research on assembly feature classification which can offer an important foundation for modeling planning and integration of design with assembly planning not even the iso produces a standard assembly feature classification although they have for manufacturing features given by step ap 224 therefore assembly feature classification was made as shown in figure 3 14 figure 3 assembly features classification assembly features provide several useful information and advantages that facilitate the process of asp for example assembly features give information on the precedence or assembly order of the components involved in the assembly of a product they represent the prediction for the next component in the assembly to select in the sequence planning they represent patterns of identical components they also represent the mating of components through a number of features connecting together simultaneously all these information result in a smaller number of assembly sequences that have to be checked in the assembly sequence optimization this can result in a significant reduction in the computation time for assembly sequence planning 13 2 methodology this paper presents a theoretical model for integration of cad with process planning and assembly planning the main modules of the framework are the cad system the assembly planning system and the process planning system each module of the system is supported by additional sub modules the integration is achieved by the data flow that carries all the needed information through the various modules of the system this integration can be used as an important step for achieving manufacturing and assembly oriented product development to reduce manufacturing costs and enhance the product quality and production efficiency using object oriented approach part and or assembly is modeled and represented by part features assembly features and their relationships based on the preliminary design information when the attribute values of the features and their relationships are determined then these features their relationships and their associated attribute values become the input to the down stream applications process planning and assembly planning systems the features contain not only the part s geometric attributes but also the information of the 211 ullah a theoretical framework for concurrent integration of cad with downstream applications manufacturing and assembly processes e g tolerance surface finish and manufacturing precedence and assembly precedence relationships the remainder of this paper is organized as follows framework for concurrent integration is provided in section 3 section 4 describes compute aided design cad module of the framework feature based design is mentioned in section 5 section 6 describes the standard for the exchange of product data step role of dfm and dfa is given in section 7 it is followed by the knowledge based module in section 8 at the end summary is given 3 framework for concurrent integration the available literature shows a lack of integration of design with the downstream activities of a product life cycle this dilutes the advantages of concurrent engineering one of the growing solutions to this problem in industries is popular dfx tools like dfm and dfa in addition to resolve the information requirements for concurrent engineering a computerized tool should be available therefore feature based design and feature conversion approaches which provide means to share common data between different domains of computerized tools are essential these key findings reveal the need of concurrent integration of design with the downstream applications the development of such a system is presented in this paper by a framework shown in the figure 4 the main modules of the framework are the cad system the assembly planning system and the process planning system these main modules are supported by sub modules like feature based modeling and pre processor process planning design for manufacture dfm assembly planning design for assembly dfa and step standard for the exchange of product model data the framework illustrates the system structure the functions of each module and the information flow between the modules future work will include the implementation of the framework to a real life problem 4 computer aided design cad cad definition was best expressed by mr john wright as he integrated the understanding of the design process with the analytical and graphic capabilities of the computer cad is a forum for the interaction of design information people and technology since its inception cad has gradually been concerned with representing increasing amounts about the objects being manipulated geometric information has moved from 2d to 3d and from planar to curved surfaces information about surface finish and color has been added geometric and topological models specify the structural relationships between components descriptions of form features can be included application specific information such as material properties or manufacturing requirements is also useful thus cad representations have moved closer to being a knowledge representation to include all aspects of knowledge about the designed object suitable additions would be precise part whole relations 31 the representation of function 26 and design rationale 19 researchers are concerned with what needs to be represented to support intcad systems and with what progress has been made in intcad systems so far 6 this module of the framework activates cad software to create a cad model for the product including drawing file and feature list in order to develop the feature based model of a part the features and their relationships are recognized and extracted from a cad data file these features and relationships carry information of great importance to the design activities of a product and are therefore of great importance for the downstream applications of a product 212 ullah a theoretical framework for concurrent integration of cad with downstream applications figure 4 a framework for concurrent integration of cad with process planning and assembly planning 5 feature based modeling it is more reasonable to construct a model in terms of meaningful entities such as holes and bosses and store that information rather than recognize them after the fact much of research work has been conducted in the approach to define a part in terms of features instead of using low level geometric entities they are referred to as design with features or feature based design 7 the design by feature or the so called feature based design fbd approach builds a part from predefined features stored in a feature library the geometry of these features is defined but their dimensions are left as variables to be instantiated when the feature is used in the modeling process feature based design provides an explicit product data in a neutral format for direct use with downstream applications 1 with feature based design used as a key component of product representation in the whole product life cycle all product development activities such as cad process planning and assembly planning will be consistent and compatible with each other for various machined parts the geometric features have specific relationships that correspond to the manufacturing and assembly operations one of the benefits of feature based design is that features can be transformed with varying degrees of difficulty into other forms of representation useful for various concurrent engineering activities concurrent automation is possible with feature based design and hence holds more promise for concurrent engineering design the implementation of the feature based design method includes feature recognition part representation and design with features feature 213 ullah a theoretical framework for concurrent integration of cad with downstream applications recognition is the understanding of a geometric model of an object to identify the relevant features and associated information feature conversion also called feature mapping is defined as the process of deriving a feature model based on a particular point of view such as machining from a feature model based on a different point of view such as design 8 in one way feature conversion typically the feature models for the analysis and planning views are derived from the feature model of the design view if the analysis or the planning results in the need to change the product model then the changes have to be made in the feature model of the design view and the feature models for the other views can be automatically updated from that model to allow the model of a product to be changed in the feature model of the view in which the need for it arises multiple way feature conversion has been developed 5 thus feature conversion is considered as an important step towards full integration in concurrent engineering an advantage of feature conversion over feature recognition is the possibility of using non geometric information stored in the existing feature model to derive the new feature model feature recognition has problems associated with the loss of data incompleteness of design and manufacturing information and the complexity and computational load of feature recognition programs 15 16 32 this module of the framework defines a part in terms of features and transforms features into other forms of representation useful for various concurrent engineering activities feature based design method identifies the relevant features and associated information in order to develop the feature based model of a part or a product the features and their relationships are recognized and extracted from an existing engineering drawing or from a cad data file it provides an explicit product data in a neutral format for direct use with downstream applications 6 standard for the exchange of product data step during the design stage of a part a designer uses a cad package and captures the characteristics of the part in a model that is wireframe a boundary or solid geometry representation of the actual part these geometric models are not suitable for retention of any kind of manufacturing features present in the part moreover the implementation of the object modeling in the cad database differs from one cad vendor to another and thus any effort to extract the geometric information from the database directly will have to be specific to that software package in other words cad files contain detailed geometric information of a part which are not suitable for using in the downstream applications such as process planning and or assembly planning different cad or geometric modeling packages store the information related to the design in their own databases structures of these databases are different from each other therefore a common or standard structure is required to be developed that can be used by all cad packages in addition the globalization of the market has increased competition requiring a reduction in time to market for the cost effective manufacturing of quality products therefore there is a great need to integrate and share information among different systems which are involved in the product development process here are many standards that support the data exchange but all of them are mainly devoted to sharing geometric information only the iso standard step iso 10303 deals with a more striving goal in transferring complete information related to the product development process 2 step standard for exchange of product model data is the outcome of a large international effort for the development of product and process model data standards for the purpose to enable data exchange between diverse computer systems and industrial applications for engineering construction and architecture thus the overall objective of the standard is to become the world wide standard for the representation and exchange of product model data 20 the step standard provides detailed assembly design representations these representations capture assembly structural and kinematics information during the design 214 ullah a theoretical framework for concurrent integration of cad with downstream applications process the model focuses on the hierarchy of the product and on the position and orientation between parts based on the step standard many new applications for product model data representation and exchange have been proposed such as tolerance analysis and synthesis of assembly 18 6 1 feature representation and data generation in the proposed framework for integration step is used because step application protocols support a wide variety of feature attributes representation of feature attributes can become quite complex making them difficult to define and understand therefore step uses the express language as a tool to represent product data in object oriented and integrated environment it is an object oriented data descriptive language that classifies and constructs product data in terms of entities it enables precision and consistency of product data representation and facilitates implementation each feature has an express representation format entity which describes the attributes associated in the definition of that feature 6 2 pre processing the pre processor is proposed in the proposed framework for integration to convert design data into a step xml data format the pre processor mechanism used in the translation process can be based on two layers the first layer known as logical layer uses express language to represent each feature in express representation format which describes the attributes associated in the definition of that feature the second layer known as physical layer defines the communication file structure called step file this file transfer mechanism represents the static aspect of step which allows exchange of product data this file is obtained by translating the express schemes defined in the logical layer to a step xml data format an object oriented approach can be implemented to facilitate the use of translation process from express format to step xml format all the express schemes defining step features can be programmed in class files each feature has an algorithm that can use the express class files to generate the required step feature data format the use of an object oriented approach for the implementation of a product model offers a logical means for representing real world objects such as machined parts therefore in this approach each feature can be programmed in a class file with its public characteristics and operations the inheritance feature of this approach can be used to define other common properties it also allows interfacing with the operations required to be performed on the instanced feature the four assembly features namely round pin hole threaded pin hole conical pin hole and rectangular pin hole shown in figure 5 are the instances of the pin hole assembly feature the pin hole assembly feature is an instance of fit assembly feature which is a particular type of assembly features the four instances of pin hole assembly feature can be further instantiated for example the instance round pin hole of the pin hole assembly feature can be further instantiated into sub instances of round pin through hole and round pin blind hole the pin hole assembly feature instance contains the properties and operations re